The present invention is directed to cryptographic communications systems, and it is directed particularly to the communication of synchronization information.
In a cryptographic communication system, the encryption/transmission and decryption/reception systems must be "in step" with each other to operate properly.
A common type of cryptographic communications system will illustrate this requirement. In this type of system, the encryption unit includes a so-called key generator, which produces a pseudo-random key stream. To generate a cipher signal, such an encryption system performs a logical operation, such as modulo-2 addition by means of an exclusive-OR gate, on the plain-text message and the key stream. The corresponding decryption system performs modulo-2 addition of the cipher text to the output of an identical key-stream generator to recover the plain text.
This type of communication system works because the two key generators, which can be set to key-variable (code) settings, are identical and so produce identical key streams when set to the same codes. For the system to operate properly, however, not only must the key streams be identical and set to the same codes, but they must be in synchronism: in deciphering a particular bit in the cipher text, the decryption unit must be at a point in the key stream the same as that at which the encryption unit was when it produced that cipher-text bit.
This synchronism requirement presents a problem because units that start out a message in synchronism can fall out of synchronism due to anomalies in the communication system, and once that happens the whole remainder of the message is lost if no steps are taken to re-synchronize. Loss of synchronization usually results from vagaries in the communication system rather than in the key generators.
In some systems, notably those of the cipher-feedback type, synchronism is automatically restored after a predetermined number of bits because of the way in which the key stream is generated, so no separate steps need to be taken to restore synchronization. But other types of keystream generators do not inherently include such an automatic re-synchronization feature and therefore need separate means of restoring synchronization. This is usually provided by transmission of synchronization signals ahead of or along with the ciphered message; the synchronization signals place the decryption-unit key generator in a predetermined point in its key stream so that it is restored to synchronization with the encryption unit.
Unfortunately, this transmission of synchronization signals is overhead: it exacts a price in bandwidth. An object of the present invention is to reduce or eliminate the bandwidth price of transmitting such synchronization information.